Allow multiple process_record() calls per scan

This is particularly relevant for, e.g., the ergodox EZ and
other keyboards with slow scan rates. Without changing the API or
behavior of individual process_record() calls, we allow a
configuration flag to make multiple calls in a single scan.

This will probably have miniscule effects on non-steno users,
and it's not enabled by default for any keyboards. Added note
about it to ergodox README.

Signed-off-by: seebs <seebs@seebs.net>

docs/config_options.md

@@ -123,6 +123,15 @@ If you define these options you will enable the associated feature, which may in
   * how many taps before oneshot toggle is triggered
 * `#define IGNORE_MOD_TAP_INTERRUPT`
   * makes it possible to do rolling combos (zx) with keys that convert to other keys on hold
+* `#define QMK_KEYS_PER_SCAN 4`
+  * Allows sending more than one key per scan. By default, only one key event gets
+    sent via `process_record()` per scan. This has little impact on most typing, but
+    if you're doing a lot of chords, or your scan rate is slow to begin with, you can
+    have some delay in processing key events. Each press and release is a separate
+    event. For a keyboard with 1ms or so scan times, even a very fast typist isn't
+    going to produce the 500 keystrokes a second needed to actually get more than a
+    few ms of delay from this. But if you're doing chording on something with 3-4ms
+    scan times? You probably want this.
 
 ### RGB Light Configuration